1. Technical Field
The present invention relates to a communication systems and more particularly to a communication system suitable for use in an information distribution system providing information from at least one server apparatus to a plurality of user terminals via a network.
2. Background Art
The Internet provides content providers with an environment capable of delivering content to users around the world directly and at a low cost. In addition, the Internet provides users an environment that allows content from around the world to be enabled for use in a standard user interface. As development, provision and use of content providing services that make use of the Internet become more prevalent, the vast amount of content available on the Internet increases daily. As a result, the ease of access to the Internet has become an important consideration in the development of content distribution services.
With the increasing popularity of the Internet, transparent system architectures employing internet technologies within LANs (Local Area Networks) have become commonplace. One basic constituent of many “Internet technologies” is the communication protocol, specifically TCP/IP (Transmission Control Protocol/Internet Protocol). In fact, a large number of networks currently employ TCP/IP.
Data communications according to TCP/IP are based on an OSI layer model (OSI Reference Model). The OSI reference model is a seven layer model used to model data relayed between a transmitter and a receiver. On the transmission side, data is relayed by sequentially adding headers for each layer to actual data from a higher layer to form packets. The packets are transmitted to the reception side. At the reception side, the transmitted packets are processed sequentially from the lowest layer (physical layer) to the highest layer. During processing in each layer, packets supplied from a lower layer are separated into data and a header corresponding to that layer. The content of the header is analyzed, and the data is handed to a next higher layer.
An example of the packet structure obtained by the processing in each layer on the transmission side shall be explained with reference to FIGS. 12-14. In this example, the transmission side and reception side are connected one-to-one using a PPP (Point-to-Point Protocol), as in general dial-up connections.
FIG. 12 shows the structure of an example TCP segment which is a packet that has undergone processing in the fourth layer (transport layer). The TCP segment is composed of a TCP header and data. The TCP header is composed of a basic header (20 bytes) and an optional header. The basic header includes information such as a source port number, destination port number, sequence number, acknowledgment number, code bits and window size. Additionally, the data is composed of actual data and a header added by processing at an upper layer equal to or higher than the fifth layer (session layer).
FIG. 13 shows the structure of an example IP datagram which is a packet that has undergone processing in the third layer (network layer). The IP datagram is composed of an IP header and data. The IP header is composed of a basic header (20 bytes) and an optional header. The basic header includes information such as a source IP address, a destination IP address, a service type, a packet length, and a protocol number. Additionally, the data is composed of actual data and a header added by processing on a layer equal to or higher than the fourth layer (transport layer) such as TCP, UDP (User Datagram Protocol) or ICMP (Internet Control Message Protocol).
FIG. 14 shows the structure of an example PPP frame which is a packet that has undergone processing in the second layer (data link layer). The numbers in parentheses in FIG. 14 are given in units of bytes. The illustrated PPP frame consists of a PPP header (5 bytes), data, and a PPP footer (3 or 5 bytes). The PPP header includes a flag, an address, a control, and a packet protocol identifier such as LCP (Link Control Protocol), IPCP (Internet Protocol Control Protocol), IP or IPX (Internetwork Packet Exchange). In addition, the data includes actual data and a header (including the above-mentioned TCP header and IP header) added by processing at least one layer equal to or higher than the third layer (network layer). The PPP footer includes an FCS (Frame Check Sequence) and a flag. The MTU indicated in FIG. 14 refers to the maximum transmission unit.
As described above, on the transmission side, the actual data to be transmitted is processed by procedures corresponding to each layer in the OSI layer model from the highest layer to the lowest layer. As such, a header corresponding to the processing of each layer is sequentially added to the actual data.
FIG. 7 illustrates an example packet 7A that has undergone all of the processes on the transmission side and is ready to be transmitted. As shown in FIG. 7, the packet 7A has a header composed of a 5-byte PPP header, a 20-byte IP header and a 20-byte TCP header. The header therefore includes a total of 45 bytes that are added at the beginning of the application data (if it is assumed that there are no optional headers). In addition, a 3- or 5-byte footer is added at the end of the application data. The size of the application data is, for example, 500 bytes and can be expanded to a maximum of 1460 bytes.
The operating sequence for performing packet communications according to TCP/IP shall now be explained with reference to the example process flow diagram of FIG. 15.
At S1, an LCP set up request message requesting set up of the LCP is sent from the data transmission side to the data reception side, or from the data reception side to the data transmission side. An acknowledgment response message (LCP Set Up Ack) corresponding to the LCP set up request is then sent from the party receiving the LCP set up request message to the other side at S2. At S3, a Challenge Message to perform identification at the other side is subsequently sent from the data reception side. Upon receipt of the challenge message on the data transmission side, a response message is sent out at S4. At S5, a Success Message to indicate that the identification on the other side has succeeded is then sent out from the data reception side to the data transmission side.
Once this sequence of operations is complete, an IPCP set up request message is sent from the data reception side to the data transmission side at S6. In addition, an IPCP set up request message is sent from the data transmission side to the data reception side at S7. At S8, an IPCP set up request message or a negative response message (Nak) is sent from the data reception side to the data transmission side. Upon receiving the IPCP set up request message, an acknowledgement response message (IPCP Set Up Ack) is sent from the data transmission side at S9.
At S10 an IPCP set up request message is then sent from the data transmission side to the data reception side. Upon receiving the IPCP set up request message, an acknowledgment response message is sent from the data reception side at S11. In this way, a PPP link is established between the data transmission side and the data reception side.
At S12, an IP+TCP Request message requesting establishment of an IP data link and establishment of a TCP connection is then sent from the data transmission side to the data reception side. Upon receipt of the IP+TCP request message, an IP+TCP acknowledgment response message is sent from the data reception side at S13. At S14, the data transmission side receives the IP+TCP acknowledgement response message and sends out a reply IP+TCP acknowledgment response message indicating that the IP+TCP acknowledgment response message has been received. In this way, a TCP connection is established between the data transmission side and the data reception side. The transmission and reception of actual data in the form of packet data is then initiated.
Packet data is first transmitted from the data transmission side by means of HTTP (HyperText Transfer Protocol) at S15. At S16, the data reception side, receives the packet data and sends out an acknowledgment response message. Then, depending on the size of the data being transmitted (e.g. the number of packets needed), the operations of S15 and 16 are repeatedly performed until the transmission of the packet data is completed.
At S17, a transmit finish message indicating that the transmission of the packet data has been completed is sent out from the data transmission side. The data reception side, upon receiving the transmit finish message, sends out an acknowledgment transmit finish message at S18. At S19 a reception complete message that the reception of data has been completed is sent out from the data reception side. Upon receipt of the reception complete message, the data transmission side sends out an acknowledgment reception complete message at S20.
In this way, the TCP session is terminated. At S21, to disconnect the PPP link, a Termination Request message requesting termination of the PPP link is sent out from the data transmission side. Upon receiving the termination request message, an acknowledgment termination request message is sent from the data reception side at S22. In this way, the PPP link is first disconnected. Upon disconnection of the PPP link, the channel is disconnected at S23 and the overall operation is completed.
In recent years, mobile communications have spread widely, and mobile data communications using mobile terminals is increasing in popularity. In the field of mobile data communications, it has become possible for a mobile user to access the Internet using a mobile terminal. Accordingly, provision for user-friendly Internet access services for mobile users is desired. Providing such mobile Internet access services using packet communications according to TCP/IP as described above, however, creates undesirable performance and operability issues.
With TCP/IP, as described above, the header of a packet is added sequentially by each layer and encapsulated. As a result, the overall header size becomes large. The header size is particularly large in comparison to the data when the actual data size is small. For example, when transferring about 500 bytes of data during mobile communications, the header size is about ten percent of the data size. Among the information contained in the header there are also fields which are left unused.
The number of signals exchanged between the data transmission side and reception side using TCP/IP is also relatively large in the operating sequences during establishment of connections prior to actually transmitting data. In the example shown in FIG. 15, a total of 14 steps from S1 to S14 are performed. Consequently, as the number of users accessing a network increases, network traffic increases dramatically and the data transfer rates may drop. In addition, since the mobile user is charged for the operations (steps S1-S14) prior to data transmission and reception, the economic burden on the mobile user may also become large.
Although these processes also occur in connections to the Internet via fixed networks, they are especially burdensome in mobile communication where data transmission capability (e.g. bandwidth) is relatively low in comparison to communication via fixed networks. In addition, since TCP/IP is relatively complex, mobile terminals with computing capability to rapidly process TCP/IP may be larger, heavier, and more expensive. Currently, mobile terminals with rapid processing capability (such as a personal digital assistant (PDA), etc.) simply include a portable computer and are therefore received well only in small markets. In contrast, mobile stations that are used for voice communication are designed for portability, operability and ease of use. The form and price of mobile stations for voice communication are believed to already be well-received in a broad market due to their general usefulness and high degree of popularity.
As mentioned above, various types of content capable of meeting the needs of various users already exists on the Internet. Due to the steady increase in the amount of content, devices that are already operated by various users such as mobile stations for voice communication are desired as devices for accessing the Internet. Thus, services for accessing the Internet using mobile stations for voice communication that have the capability to rapidly process TCP/IP will be well-received in a wide market. Even without taking into consideration the problem of data transmission capability in mobile communications.
One might also consider having the content providing side develop content that is customized to the data processing power of mobile stations for voice communication, and/or the data transmission capability of mobile communications. The development of this type of customized content may place a heavy burden on the content providing side. Accordingly, it is predicted that only a small amount of content with uses that are restricted to mobile stations for voice communication as compared to the content of the Internet will be available to the user.
From the above description, it is believed that the foundation of mobile data distribution is in the combination of mobile stations with voice communication with the Internet. In order to achieve this combination, it is necessary to develop efficient communication technologies which enable mobile users to effortlessly use content from the Internet using mobile stations with voice communication.